The original Hellsing anime was made long before the manga had finished, so, the writers created something entirely differant from the manga's original plotline from around....the half-way point or so.

Hellsing Ultimate is a complete recreation of the manga, following the storyline and basically bringing the manga to life. The manga just recently ended too. So...10 years ago was Hellsing. Now, and much prettier, comes Hellsing Ultimate. That's the basic differance between the two.

For the REAL BIG differances, there's one word. Nazis. Ultimate like the manga follows the impending war with a group of leftover Nazis who discovered and began creating fake vampires. Then there's the Vatican's Section XIII every now and then but basically that's it.

...I personally like the Nazis better then the weird dude from the original...what was the naked guy's name?....anywho, yeah. I love the nazis and I love Ultimate to pieces.